Comparison of the PN N = 2–1 and HDCO J = 2–1 flux distributions. The PN N = 2–1 line flux (colourscale and bluecontours) is integrated in the velocity interval [−11;+4.6] km s⁻¹; the first contour and contour interval are 3σ (2.8 × 10⁻² Jy beam⁻¹ km s⁻¹) and 2σ (1.8 × 10⁻² Jybeam⁻¹ km s⁻¹), respectively. The synthesized beam of the PN N = 2–1 is depicted by a blue ellipse in the bottom right corner. The HDCO J = 4_1,4−3_1,3 emission (white contours) is integrated in the velocity interval [−0.7;+3.4] km s⁻¹; the first contour and contour interval are 20% and 15% of the peak intensity, respectively. The map was smoothed down to the angular resolution of the PN N = 2–1 line. The HDCO J = 2–1 data are from Fontani et al. (2014). Coordinates are in arcsecond offsets with respect to the nominal position of L1157-B1.